1. Field of the Invention
The present invention relates to a transmission system comprising a first station connected to a second station by a transmission channel, each station comprising a transmitter and a receiver, at least the first station comprising an echo canceller arrangement which includes an adaptive filter for generating, on the basis of data transmitted by the transmitter, a replica of an echo present in the input signal of the receiver, and a subtracter circuit for supplying to the receiver the difference between the input signal and the echo replica. The adaptive filter is arranged for determining the echo replica by calculating a sum weighted with weight factors of the transmitted data as delayed by multiples of a time period T.
2. Description of the Related Art
Such echo canceller arrangements are well known and find important applications in the field of telephone transmissions and data transmission. The echoes are most frequently due to mismatches caused by the 2-wire/4-wire transitions situated between a near-end speaker and a far-end speaker. In the field of telephone transmissions, the more distant the echo is, the more noticable it becomes. This effect of distance is enhanced in the digital teIephone systems, which may introduce additional delays caused by the fact that a certain period of time is devoted to packetizing the speech signals.
For adjusting the weight factors one often uses the sign method. This method is described, more specifically, in paragraph IV.7 of the article by M. BELLANGER entitled: "ANALYSE DES SIGNAUX ET FILTRAGE NUMERIQUE ADAPTIF", published in 1989 by MASSON, PARIS. This method, which utilizes nonstationary signals, such as speech signals does have some drawbacks, however. It corrects the factors by adding thereto or subtracting therefrom a fixed quantity. This value is either low, and so the convergence of the factors to their optimum value is slow, or is high and so there is a risk of instability.